In recent years, electronic apparatuses such as AV apparatus and personal computer, communication apparatuses, and the like, have been rapidly becoming more portable and cordless. For power sources of such electronic apparatuses and communication apparatuses, secondary batteries having high energy density and excellent load characteristics have been demanded. Use of lithium secondary batteries having high voltage, high energy density, and excellent cycle characteristics has been expanding.
However, a conventional lithium secondary battery generally uses an electrolytic solution as an electrolyte. Since organic solvents constituting these electrolytic solutions are flammable and have risk of ignition, they have problem about safety.
Since a so-called solid battery using a solid electrolyte as an electrolyte does not use a flammable electrolytic solution, it has high safety and can achieve theoretically high energy density, and therefore it has been studied by many universities, manufactures, etc.
However, in the solid battery, since not only an electrode but also an electrolyte is solid, contact portions on the interface between particles forming the electrode and particles forming the electrolyte become smaller, thus making movement of lithium ions or electrons difficult as compared with the case where an electrolytic solution is used as electrolyte. Then, the interface resistance is increased, and, as a result, battery characteristics such as energy density tends to become lower.
As a method for suppressing the interface resistance between a solid electrolyte and an electrode, a method of sandwiching an interface layer made of a mixture of electrolyte particles and electrode particles between the electrolyte and the electrode, or a method of coating surfaces of the electrolyte particles and the electrode particles with a conductive coating film, and the like, have been proposed. However, substantial reduction of the interface resistance cannot be achieved.
Meanwhile, since sulfur has a very high theoretical capacity density as 1675 mAh/g, and can be expected as a battery material having high energy density, lithium-sulfur batteries using sulfur as a positive electrode active material, and lithium metal as a negative electrode have been considered.
However, also in the case of the lithium-sulfur battery, when a solid electrolyte is used as an electrolyte, as mentioned above, the energy density of a battery is not as high as expected due to interface resistance generated in the interface between the electrolyte and an electrode.
Furthermore, in a case where an electrolyte containing an organic solvent is used, in addition to risk of fire, there is a problem that a sulfur molecule or a reaction intermediate (for example, lithium polysulfide) generated by the reaction between a lithium ion with sulfur is dissolved and diffuses into an electrolytic solution during charging and discharging, thus causing occurrence of self-discharge or degradation of a negative electrode. Use of an ionic liquid as an electrolyte can avoid the risk of fire, but it cannot prevent the sulfur molecule and the polysulfide ion from being dissolved, and may cause deterioration of the battery performance.
As a production method for a battery electrode, PTL 1 proposes a method of forming an active material layer by attaching a paste to a current collector, in which the paste is obtained by heating and reducing pressure treatment with a mixture of electrode active material and an ambient temperature molten salt. The ambient temperature molten salt is a combination of cation components including imidazolium cations such as ethylmethylimidazolium tetrafluoroborate, ammonium cations such as diethylmethylpropylammonium trifluoromethanesulfonylimide, and pyridinium cations such as ethylpyridinium tetrafluoroborate, and anion components including tetrafluoroborate anion (BF4−), hexafluorophosphate anion (PF6−), trifluorosulfonyl anion ((CF3SO2)2N−), and bis(trifluorosulfonyl)imide anion ((C2F5SO2)2N−). To a liquid electrolyte in which supporting salt (lithium salt) is added to the ambient temperature molten salt, powder of lithium cobalt oxide, lithium nickel oxide, lithium nickel cobalt oxide, and lithium manganese oxide are mixed as the positive electrode active material.
However, in the lithium-ion secondary battery described in PTL 1, a positive electrode active material layer and a negative electrode active material layer are stacked so as to face each other with a separator sandwiched therebetween, and the stack is impregnated with the ambient temperature molten salt electrolyte to produce a coin-type lithium-ion secondary battery. Therefore, the lithium-ion secondary battery is not an all-solid-state lithium secondary battery using a solid electrolyte as an electrolyte.